Ethylene polymers are used for various uses such as films, sheets, microporous membranes, a fibers, foam bodies and pipes. Ethylene polymers are used because melt-processing of them are easy and molded articles obtained therefrom have high mechanical strength and are excellent also in chemical resistance, rigidity and the like. In particular, ultrahigh molecular weight ethylene polymers have large molecular weights and hence have higher mechanical strength, are excellent in slidability and wear resistance and are excellent also in chemical stability and long-term reliability.
The ultrahigh molecular weight ethylene polymers are, however, low flowability even when crystals thereof are melted at a temperature equal to or higher than its melting point, and hence melt-molding cannot easily be performed. Therefore, processing methods in which ultrahigh molecular weight ethylene polymer powders are filled in molds, the resultants are compressed at high pressure to produce molded article blocks, and the blocks are cut into film or sheet shapes are generally employed (see, for example, Patent Literature 1). As another processing method, a method in which the ultrahigh molecular weight ethylene polymers dissolved in a solvent is processed and then the solvent is removed therefrom is selected. For example, microporous membranes, high-strength fibers and the like are heated and kneaded together with a solvent such as liquid paraffin or decalin by an extruder to be processed into membrane or fiber shapes while cooling. Thereafter, the solvent is removed by extraction or the like, and the resultant is heated again up to the vicinity of the melting point for stretching or heat setting (see, for example, Patent Literature 2).
In this manner, the ultrahigh molecular weight ethylene polymers are finely processed by repeating melt and solidification with the temperature highly accurately controlled, so that physical properties of resultant molded articles can be controlled. In using an ethylene polymer, that is, a semi-crystalline resin, the physical properties of molded articles are largely affected by the degree of crystallization, a crystal size, molecular orientation and the like, and a rate of the ethylene polymer crystallizing in accordance with temperature change is extremely significant.
As a known publication describing control of the rate of crystallization, for example, Patent Literature 3 discloses that if an isothermal crystallization time at a temperature lower by 25° C. than a melting point of a polyolefin resin is 200 seconds or less in a microporous membrane, crystallinity of a starting material and a film-forming property at a low draft ratio are so well balanced that raw film formation can be easily conducted.
Besides, for example, Patent Literature 4 discloses that for a microporous membrane, when a crystal nucleating agent is added to a polyolefin resin composition containing a polyolefin resin and a film forming solvent, a crystallization time is shortened, a porous structure of the resultant microporous membrane becomes more uniform and denser, and the mechanical strength and withstand voltage characteristics thereof are improved.